As an element or device, which has a characteristic such that the resistance value increases as the temperature rises (PTC characteristic), an organic positive thermistor is known. The organic positive thermistor is widely adopted as an element for an over current protection circuit, a self-control type heating element, a sensor element for temperature detection and the like.
Generally, the organic positive thermistor has such a structure that a conductive polymer, in which an electric conductor is dispersed in a resin and molded, is sandwiched by, for example, an electrode couple having a plate-like shape. As an example of such conductive polymer, a conductive polymer disclosed in the specification of U.S. Pat. No. 4,237,441, in which a specific amount of fine particles of carbon black as the electric conductor is mixed in a crystalline polymer as a matrix resin, is given.
In a load equipment or circuit to which an organic positive thermistor, in which such conductive polymer is employed, is connected, during operation in a normal state under appropriate temperature conditions, a steady-state current flows through the thermistor. This owes to the fact that the conduction is ensured by many fine particles, which are joined with each other, as the electric conductor included in the resin.
Contrarily, when there occurs an abnormality such as an operation under a non-normal state like overload etc or a short circuit within a device, an over current flows through the thermistor. In such a case, the conductive polymer is heated due to the over current. In the case where the polymer is a thermoplastic resin, it is conceivable that the resin is softened, melted, or fused resulting in an inflation in the volume of the resin. Accompanying this, the connection among the conductor particles is released and the conductive path is gradually cut off. Thus, the PTC characteristics are made to function.
Also, when a thermosetting resin is used as the polymer, when the temperature reaches to a deformation temperature of the resin or a glass-transition temperature due to the heat by the over current, the conductive path formed of many conductor particles is likewise cut off.